Hydrocracking is an established process in petroleum refining and in its commercial scale operation zeolite based catalysts are progressively gaining market share because of their higher activity and long term stability. Large pore size zeolites are conventional for this purpose, for example, zeolite X or the various forms of zeolite Y such as ultrastable zeolite Y (USY). Another zeolite which has properties consistent with those and which has been described as having a structure comprising the 12-rings characteristic of large pore size zeolite is zeolite beta and this zeolite has been proposed for use as a hydrocracking catalyst in EP 94827. Zeolite beta is notable for its paraffin-selective behavior. That is, in a feed containing both paraffins and aromatics, it converts the paraffins in preference to the aromatics. This phenomenon is utilized in the hydrocracking process disclosed in EP 94827 to effect dewaxing concurrently with the hydrocracking so that a lower bottoms product pour point is achieved concurrently with a reduction in the boiling range. Another application of the properties of zeolite beta is to dewax petroleum feedstocks by a process of paraffin isomerization, as opposed to the selective paraffin cracking produced by the intermediate pore size zeolites such as ZSM-5. This dewaxing is disclosed in U.S. Pat. No. 4,419,220 and an improvement on the basic zeolite beta dewaxing process is described in U.S. Pat. No. 4,518,485 in which the feedstock is first subjected to hydrotreating in order to remove heteroatom-containing impurities such as sulfur and nitrogen compounds prior to the isomerization reaction. During the hydrotreating process the organic sulfur and nitrogen containing compounds are converted to inorganic sulfur and nitrogen, as hydrogen sulfide and ammonia respectively. Cooling of the hydrotreater effluent and interstage separation between the hydrotreating and dewaxing steps enables the inorganic nitrogen and sulfur to be removed before they pass into the catalytic isomerization/dewaxing zone.
From this discussion it is clear that zeolite beta based catalysts may, under appropriate conditions, promote isomerization reactions in preference to cracking reactions or, under other conditions, cracking reactions over isomerization reactions. The balance between the various types of reactions which may occur is dependent upon a number of factors including the composition of the feed and the exact process conditions which may be used. In general, cracking reactions are favored by the use of higher temperatures and more acidic catalysts while isomerization reactions are favored by lower temperatures and the use of a hydrogenation/dehydrogenation component on the catalyst which is relatively active. Thus, isomerization tends to be favored by the use of a catalyst containing a noble metal such as platinum which is highly active for hydrogenation and dehydrogenation reactions, a zeolite which has a moderate acidity and the use of moderate temperatures.
Although these considerations indicate that it would be possible to carry out the desired types of reactions in a selective manner by varying the composition of the catalyst in accordance both with the feedstock available and the desired product, life in the refining industry is rather more difficult outside the laboratory. In a refinery, loading and unloading of catalysts from a reactor is an expensive and time consuming process and is to be avoided if possible. Similarly, feedstocks of the desired composition may not always be available and the product characteristics may change from time to time, depending on the demand for them. Thus, the realities of commercial refining require that a process should be capable of ready adaptation to different feedstocks and different product demands with the minimum of operating changes: in particular, catalyst changes should be avoided if possible. For these reasons, it would be desirable to find some means of modifying the activity and product selectivity of the zeolite beta and other zeolite catalysts so as to modify the yield structure of the catalyst and hence, of the process in which it is being used. If this could be done, it would be possible, for example, to process different feedstocks so as to effect a bulk conversion as well as a dewaxing or, alternatively, to carry out dewaxing by isomerization or to alter the selectivity to distillate or naphtha hydrocracking products. In the first case, waxy gas oils could be hydrocracked and dewaxed at the same time to produce low pour point distillate products such as heating oil, jet fuel and diesel fuel and in the second case, lubricant feedstocks could be selectively dewaxed by isomerization.
Another aspect of the use of zeolite based hydrocracking catalysts such as zeolite X and zeolite Y which is of some importance in the refining industry is that they have a potential for temperature runaway under adiabatic reaction conditions, which may cause irreversible damage to the cracking catalyst and process equipment. Recent studies have shown that the high activation energy for zeolite-catalyzed hydrocracking process coupled with a relatively high hydrogen consumption, suggests that temperature runaway is highly plausible for a hydrocracker using a zeolite-based catalyst. The potential for harmful unexpected exotherms is particularly great when conditions are changed e.g. feed composition is altered. In addition, excessive exotherms may arise under steady state conditions: the temperature at some point in the reactor--usually the back end, may be stable but too high for the desired degree of selectivity or cycle length.
Currently available schemes for controlling temperature runaway utilize quench hydrogen to lower the reactor temperature in the high temperature stage. Hydrogen quench is effective for a normal operation with minor adjustment of reactor temperature but under potential temperature runaway situations hydrogen quench may be disastrous. This is partially due to the injection of additional hydrogen to the "hydrogen starvation" temperature runaway zone. Another factor which has often been ignored is the wrong way behavior, resulting from the differences in the creeping velocity between mass and heat transfer waves. See "Chemical Reactor Design and Operation," Westerterp, Van Swaaij, and Beenackers, John Wiley & Sons, 1984. The injection of the quench hydrogen reduces the temperature and conversion near the inlet of the potentially dangerous stage. Under normal conditions, heat waves travel slower than mass waves. Consequently, the high temperature zone, which normally appears near the outlet of the stage for an adiabatic reactor, may be fueled with unconverted hydrocarbons entrained from the quenched zone. Eventually, the reactor will attain its lower temperature steady state. However, this dynamic response of the wrong way behavior using hydrogen quench may potentially induce irreversible deactivation for the cracking catalyst, e.g., sintering of the metal hydrogenation component. Damage to the process equipment e.g. reactor and heat exchanger, resulting from the wrong way behavior, is possible. For this reason some alternative method of controlling hydrocracker operation including, in particular, temperature excursions, is desirable.